Radio waves which propagate through the atmosphere typically have regions of relatively low signal strength resulting from destructive interference, shielding, etc. In many applications calling for the reliable transmission and reception of physiological telemetry such as ECG, EEG, and EMG data of human and nonhuman subjects, such dead spots present a possibility of an intolerable loss of valuable diagnostic and research data. Where the source is fixed, it is a relatively easy job to avoid placing the receiving antenna in a dead spot. With moving sources, as are found in telemetry of physiological data from ambulatory subjects, the appearance of a dead spot at the receiving site from time to time is almost a certainty.
It is known that if two or more receiving channels are sufficiently separated in space, the fading on the various channels is more or less independent. One known spatial diversity receiving system that makes use of this fact to maintain received signal level typically employs plural spaced-apart receiving antennas, receivers and detectors, and means operative in response to the signal-to-noise level of the radio frequency energy at each antenna to select the detected output from the channel having the strongest level. However, undesirable noise is produced in the selected output signal from switching-induced transients. In addition, in the case where each independent channel employs FM demodulation, different DC bias levels are commonly produced in the corresponding independent channels that introduces undesirable noise as the diversity receiver switches between the channels in dependence on the strength of the individual output signals therefrom.